Effects of menthol-based counterirritant on quadriceps motoneuron-pool excitability.

CONTEXT Motoneuron-pool facilitation after cryotherapy may be mediated by stimulation of thermoreceptors surrounding a joint. It is unknown whether menthol counterirritants, which also stimulate thermoreceptors, have the same effect on motoneuron-pool excitability (MNPE). OBJECTIVE To compare quadriceps MNPE after a menthol-counterirritant application to the anterior knee, a sham counterirritant application, and a control treatment in healthy subjects. DESIGN A blinded, randomized controlled laboratory study. SETTING Laboratory. PARTICIPANTS Thirty healthy subjects (16 m, 14 f; 24.1 +/- 3.9 y, 170.6 +/- 11.4 cm, 72.1 +/- 15.6 kg) with no history of lower extremity surgery volunteered for this study. INTERVENTION Two milliliters of menthol or sham counterirritant was applied to the anterior knee; control subjects received no intervention. MAIN OUTCOME MEASURES The average vastus medialis normalized Hoffmann reflex (Hmax:Mmax ratio) was used to measure MNPE. Measurements were recorded at 5, 15, 25, and 35 minutes postintervention and compared with baseline measures. RESULTS Hmax:Mmax ratios for all groups significantly decreased over time (F(4,108) = 10.52, P < .001; menthol: baseline = .32 +/- .20, 5 min = .29 +/- .18, 15 min = .27 +/- .18, 25 min = .28 +/- .19, 35 min = .27 +/- .18; sham: baseline = .46 +/- .26, 5 min = .36 +/- .20, 15 min = .35 +/- .19, 25 min = .35 +/- .20, 35 min = .34 +/- .18; control: baseline = .48 +/- .32, 5 min = .37 +/- .27, 15 min = .37 +/- .27, 25 min = .37 +/- .29, 35 min = .35 +/- .28). No significant Group x Time interaction or group differences in Hmax:Mmax were found. CONCLUSIONS Menthol did not affect quadriceps MNPE in healthy subjects.